colour changes while viewing pdf and when exporting to png

I've got a number of pdfs that change between bright and dull colours while I'm zooming in and out of the pdf. I'm using Acrobat Standard Pro.

Printing the pdf to paper results in the normal bright colour scheme. However, when exporting to .png I get the dull colours. Unfortunately, I really need the png-files in the bright colours...

Has anyone experienced a similar issue? Is there a fix for this?

Any help would be much appreciated.